“Our emails are that central part of our online identity,” said cyber security expert Adam Doupe. “When you go to a website and forget your password, the first thing you do is click that ‘forget password’ link, which sends an email to your account with a link to change your password, so once someone has access to you email, they have access to all the websites you have an account on."

The data stolen may have included names, passwords, telephone numbers and email addresses.

The company says the investigation shows no signs of the hacker in its system currently.